Retinal pigment epithelial (RPE) cells and the photoreceptor (PR) cells are functionally and developmentally closely integrated. During development of the outer retina in rat, the determinant recognized by the RPE-specific monoclonal antibody RPE-9 is first expressed at post-natal day 3 whereas the PR outer segments (OS) appear at day 5. The OS first appears where RPE cells are already expressing their determinant. RPE-9 recognizes a 67-kDa protein specific to the RPE. This protein is found in mammalian and avian RPE. A membrane-associated protein, it is probably non-glycosylated. We have begun to screen a bovine RPE cDNA library for a cDNA for this protein. We have subcloned DNA fragments corresponding to the first two repeats of bovine IRBP into a bacterial expression vector. IRBP is involved in the transport of retinoids, a functional relationship between the RPE and the PR. The resultant expressed protein fragments will be tested for their ligand-binding and immunological properties.